Lubricating oils as used in, for example, the internal combustion engines of automobiles or trucks are subjected to a demanding environment during use. This environment results in the oxidation of the oil catalyzed by the presence of impurities in the oil, such as iron compounds, and is also promoted by the elevated temperatures experienced by the oil during use. This catalyzed oxidation of the oil not only contributes to the formation of corrosive oxidation products and sludge in the oil but can also cause the viscosity of the oil to increase or even cause the oil to solidify. This oxidation of lubricating oils during use is usually controlled to some extent by the use of antioxidant additives which may extend the useful life of the oil, for example, by reducing or preventing unacceptable viscosity increases.
Aminic antioxidants are antioxidants that contain one or more nitrogen atoms, such as alkylated diphenyl amines and phenothiazines. Phenolic antioxidants contain one or more sterically hindered phenol functionalities, and can be either used alone or in synergistic combinations with alkylated aminic antioxidants. The synthesis and uses of phenolic antioxidants, phenothiazines and aromatic amines have been reported. Phenothiazine antioxidants have been used as a stand alone additive, chemically modified or grafted onto the backbone of polymers.
There is, however, a continuing need for new antioxidants and antioxidant systems which offer improved performance and which are effective at low levels. There are a number of factors which have contributed to this continuing need. One such factor is that in recent years internal combustion engines are often operated at even higher temperatures, which tend to increase the rate of oxidation and shorten the useful life of the oil. In addition, there is a strong desire to use cheaper base stocks for lubricating oil compositions which have inferior resistance to oxidation and require more efficient and effective antioxidants. There is also a need for lubricating oils to have a longer in service life span to support the longer service intervals for motor vehicles. There is also a desire to find antioxidants and antioxidant systems which meet the above requirements and at the same time are not detrimental to other aspects of motor vehicle performance. In this respect there is a desire for antioxidants which do not contribute to the phosphorus content of motor vehicle exhausts, as phosphorus is detrimental to the performance of catalyst based exhaust purification systems. The trend to reduce phosphorus levels in the final formulation has led to use of lower levels of zinc dialkyldithiophosphates, (ZDDP). This has led to an overall reduction in the levels of antioxidants used in the final formulation because ZDDP also serves as an antioxidant, in addition to an extreme-pressure/antiwear additive. The trend to reduce the total levels of sulfur in lubricants will also lead to lower use levels of sulfur containing multifunctional antioxidant extreme-pressure additives such as sulfurized olefins, and other sulfur containing detergents. In addition some antioxidants, such as for example diphenylamines, cannot be used at relatively high concentrations as this may result in sedimentation or deposits in hot engine areas such as the diesel ring areas in diesel engines. The invention is concerned with the problem of providing an improved antioxidant for use in lubricating oils.